1. Field of the Invention
This invention relates to a cellular base station communication system and more particularly to a panel antenna having a compact stackable variable phase shifter.
2. Description of Related Art
Differential variable phase shifters introduce a desired phase shift between RF energy split between two or more outputs. Differential variable phase shifters are useful, for example, as components in the electrically variable beam elevation and or azimuth scan angle antenna systems of cellular communications base stations. The desired phase shift is typically obtained by modifying the electrical path required to reach each output with respect to the other output(s). To adjust the electrical path in one common design approach, a transmission line conductive arc has an associated wiper, pivoted at the center of the arc, which is moved along the surface of the arc, apportioning the length of an electrical path from the wiper input to either end of the conductive arc depending upon the position of the wiper along the conductive arc.
The wiper has a conductive component to transmit the input signal to the conductive arc. In typical prior art differential phase shifters of the capacitive pivoted wiper type, a non-conductive dielectric element is used between the conductive arc and wiper conductive component to reduce inter-modulation distortion (IMD).
As will be described below, the wiper may be an arm composed of metal; in that approach the arm comprises the wiper conductive component. The dielectric element in the metal wiper design arrangement is typically a dielectric shim, for example.
Alternatively, the wiper may comprise, a microwave quality dielectric material having a conductive trace on its surface facing away from the conductive arc and groundplane behind the arc. If the spacing between the wiper conductive element and the conductive arc varies significantly as the wiper is pivoted along the conductive arc, the capacitive coupling between the two conductors will vary, causing undesired variations in both reflected and transmitted energy.
The spacing variations may be caused, for example, by the wiper being coupled too loosely to the conductive arc. On the other hand, if the wiper is pressed too firmly against the trace, the wiper may bind or require excessive force to move.
In addition, some method of transferring motion to the wiper from a point external to the phase shifter is needed to allow remote adjustment of the wiper location along the conductive arc. The remote adjustment linkage device is preferably non-conductive in nature so as to avoid distorting the EM fields in the phase shifter and to avoid generating IMD.
In the current art these various functions of providing mechanical wiper support and remote position adjustment are accomplished with multiple parts which undesirably increase the size, cost, complexity, and reliability of the overall structure. In one embodiment alluded to above, the conductive arc and wiper are formed of cast, stamped or formed metal. Non-conductive spacing shims or sheets are used to improve IMD performance. Additional non-conductive plastic parts are typically added to connect the wiper to the remote adjustment linkage device. Additional non-conductive fasteners and/or spacers are typically used to support the arc and metal wiper and to hold them in close contact.
In another embodiment mentioned above the wiper body is a substrate composed not of metal, but rather of a dielectric material, and the wiper conductive component is formed as a conductive trace upon a dielectric substrate. The trace is located on the substrate surface facing away from the arcuate conductor. Because the wiper conductive component comprises part of the transmission line to the radiating elements, in this prior art approach the dielectric wiper body must be composed of a microwave-quality dielectric substrate such as PTFE or PTFE-ceramic glass fiber laminates. Such microwave-quality substrates are electrically distinct from standard printed circuit board (PCB) substrates such as epoxy-glass in two ways. They exhibit much lower insertion loss at RF frequencies and they exhibit much tighter tolerance in their dielectric constant. Depending upon the electrical characteristics and uniformity required, microwave-quality substrates may cost between 3 to 100 times more per square foot than standard printed circuit board substrates.
Using a PCB substrate for the wiper element has a number of advantages. The first is that the dielectric substrate can be used as the non-conductive layer between the arc conductor and wiper conductor. The second is that the wiper substrate, being non-conductive, can be extended beyond the phase shifter to act as a lever arm for connecting the wiper element to a phase shifter adjustment linkage external to the phase shifter.
However, if the dielectric substrate is located between the arc conductor and the wiper conductor, then it must be of microwave quality. This causes several problems. One is that extending the wiper substrate to attach to the linkage is not economically desirable due to high cost of the material relative to other plastics. Secondly, most microwave-quality substrates lack the structural stiffness required for use as a mechanical support member. Therefore, most implementations that utilize microwave-quality substrates add additional mechanical elements, such as bars or springs in order to maintain the proper spacing between the arc conductor and wiper conductor and to provide the necessary structural support for the wiper.
“Antenna System”, U.S. Pat. No. 6,573,875 issued Jun. 3, 2003 to Zimmerman et al, hereby incorporated by reference in the entirety, describes a phase shifter implementation upon the back plane of a cellular base station radiator array antenna using microwave quality substrates for the wiper as described herein above. To adjust the phase between five radiator clusters of the antenna, two separate phase shifter modules with a common adjustment linkage are applied. Each five output phase shifter module is adapted for minimum front-to-back thickness (height) to allow the host antenna to have a minimum height profile for reduced wind loading and improved visual impact.
Reductions in wind loading allow an overall reduction in the structural requirements of the antenna system as well as those of the mounting hardware and support structure, thereby reducing overall costs. Visual impact is an important consideration due to growing public resistance to the addition of obtrusive antenna structures to existing buildings and or installation of new antenna towers on esthetic grounds.
Resulting antenna thickness prevents desired use of a single centrally located stacked phase shifter assembly. To achieve the desired minimum thickness or height of the overall antenna, individual phase shifter modules with five outputs each are placed end to end and linked together by a common mechanical linkage adapted to be as thin as possible. As a result, the phase shifters take up a significant portion of the antenna backplane surface area. Cabling from the phase shifter outputs to each of the desired radiator clusters is manufactured with identical lengths of coaxial cable for manufacturing and design simplification whereby further phase adjustments do not occur after the phase shifter(s) because the final connection to each radiator has an identical length, i.e. the length of the longest path. However, because the phase shifter(s) are covering a large portion of the antenna back plane, the longest path from each phase shifter module is significantly increased. Also, because the mechanical linkage must extend to each wiper arm, the mechanical linkage includes a plurality of individual components such as link arms and fasteners.
Other antenna systems incorporating phase shifters have stacked phase shifter printed circuit boards upon each other and combined arc traces with a common wiper arm to reduce linkage complexity and the longest length of the interconnecting radiator cables. However, the stacked configurations significantly increase the overall thickness or height of the resulting antenna and enclosing radome.
Another prior configuration applies a stacked wiper configuration positioned on the radiator side of the backplane. This configuration may reduce the overall thickness or height of the antenna but may cause anomalies in the antenna radiation pattern(s) as well as increases in linkage complexity and or the total number of required manufacturing operations.
Competition within the antenna system and phase shifter markets has focused attention also on improved electrical performance, reliability, ease of use and materials and manufacturing operations costs.
Therefore, it is an object of the invention to provide an apparatus that overcomes or ameliorates the described deficiencies in the prior art.